Copeland Scroll, Liquid returning

[Collie]

Hi lads, Having a problem with an LG split AC unit running on 407c. When I ran the unit up, I noticed the head pressure was very low, only 6bar(g), with a good back pressure of about 4bar(g), very little heat from the condenser.

The discharge of the compressor was warm but it was comfortable to touch.
The 4way valve is ok, its not passing. After a few minutes of running i could see the suction pipe icing up from the service valve at the side of the unit all the way back to the compressor.

I checked indoor unit found good airflow and very good cooling occurring. I decided to switch to heat mode just to see what it would do.
Whenit ran in heat the head pressure shot up from a standing pressure of 5bar(g) to over 30bar(g)! in the space of about ten seconds!! So my compressor is definitley pumping.

I read somewhere that copeland scrolls seperate when they try to compress liquid, so if say i had liquid returning, the scrolls seperated and bypassed the compression process, would this give my my low head pressure???
Any thoughts guys? thanks

[chemi-cool]

Frost on suction line does not necessary means liquid returning.
Check compressor currant draw, sound to me as you need to replace compressor.

[multisync]

First thing to do would be to check the charge as it sounds more like an overcharge or possibly indoor fan/filter problem.

I don't want to insult your intelligence but I wonder if the discharge of 6 bar is actually the pre-expansion? Did you get onto the actual discharge or just used the two ports on the service valves?

[monkey spanners]

What Multisync said, also if it was pumping liquid it would sound growly and you would't want to be stood near it.
Has it got a low ambient kit/fan speed control on the outdoor unit? If not it could be overcondensing which would lead to a low evaporating temp.
Superheat measurement is the way to check for liquid returning to the compressor.

[andy c]

It sounds to me, like you have a partial blockage at the service valve. Plastic end caps left in the copper, whilst installing the pipework are the usual cause. But don't quote me on that.

[Thermatech]

Fixed speed compressor?
If the system needs to be cooling in December then I would suspect it is installed in some computer/ comms room & in this application a standard comfort cooling split system can run in frost condition on the indoor coil / suction line due to the high sensible load which the system is not designed for.
Some cheap small split systems have no low ambient / HP control fitted as standard so the outdoor fan runs flat out & the unit is overcondensing with very low discharge pressure. This tends to make the evaporating pressure / temp also very low.

This type of problem has been well documented a number of times before on this site.

Try making the system run at higher discharge pressure by reducing air flow through the outdoor coil & get the discharge pressure up around 20 to 25 bar.
If the system has correct refrigerant charge & no other problem like blockage then as you force up the discharge pressure up you will see improved suction temperature with a target of getting the system to run with no frost at the indoor unit coil or suction line.
If this workes then you can investigate fitting a suitable low ambient control to the system as a permanant solution.
The unit could be designed for cooling in up to 45 deg C ambient so the outdoor coil is oversized for cooling in ambient conditions you have in December.
Some engineers even take out one fan motor on twin fan outdoor units & just fit low ambient fan speed control to the one remaining fan to prevent overcondensing for low ambient cooling applications.

The frosted suction would tend to indicate some liquid return & this will kill the compressor in time due to liquid refrigerant mixed with the oil in the compressor shell which damages the bearings over time.

[Collie]

Thanks for all the responses guys, got it sorted. Weighed out the charge and it was quite a bit over what it was meant to be. And the high pressure problem in heating mode was a fairly severe twist in the pipe, just after the 4way valve.